Polymeric matrices, including porous matrices and membrane matrices, are well known in the art. Membrane matrices are used in a variety of filtration applications, such as purification and testing in the food and beverage industry, water treatment, pharmaceuticals, and in medical laboratories. Porous matrices have become increasingly relevant to the testing industry for uses including medical diagnostics, e.g., glucose monitoring test strips.
Most polymeric matrices are generally made by first preparing a casting solution made up of the chosen polymer in a suitable solvent. The casting dope is then formed into a thin sheet and the polymer is precipitated or coagulated into a solid phase. Precipitating or coagulating the polymer into a solid porous matrix or membrane matrix is normally carried out by evaporating the solvent or contacting the polymer with a non-solvent liquid in a coagulation bath. By varying the composition of the polymer or casting solution, or the process conditions, matrices having varying morphology, porosity, and performance characteristics are produced.
In producing the matrices of the present invention, various technologies come into play. These include the dissolution of polymers in sulfonating solvents, the use of sulfonated polymers to make matrices, and the use of acid-type solvents for making matrices.
The dissolution of polymers, including aryl ether ketone, aryl sulfonates and aramid type polymers (e.g., polysulfone, polyethersulfone, polyetherketone, poly-paraphenylene terephthalamide), in sulfonating solvents has been disclosed. For example, phase diagrams for Kevlar™ (poly-paraphenylene terephthalamide, or PPTA) in sulfuric acid are available in “Polymeric Materials Encyclopedia” from CRC Press. The original method for the making of Kevlar™ fibers involved the dissolution of the polymer in concentrated sulfuric acid. The problem of the degradation of the material in sulfuric acid is discussed in the literature in general.
The use of sulfuric acid on a polymer to induce such a “degradation”, i.e. to sulfonate the polymer, is reported in patents assigned to Akzo and ICI (U.S. Pat. No. 4,992,485 and DE-3321860). These patents disclose a process for preparing a sulfonated polymer, which is collected after sulfonation for further use. In U.S. Pat. No. 4,419,486 (EP-8894), a hydrophilic sulfonated polyaryletherketone polymer is derived by controllably sulfonating a copolymer. The sulfonation may be effected using 98% w/w concentrated sulfuric acid at an elevated temperature. The sulfonation of aromatic polyether sulfones by means of sulfur trioxide in concentrated sulfuric acid as solvent is disclosed in U.S. Pat. No. 5,013,765 (EP-341473). Side reactions and degradation reactions may be suppressed to a large extent by maintaining a sulfur trioxide content of less than 6 percent by weight, based on the solvent, and a reaction temperature of less than 30° C.
The use of sulfonated polymers to make membranes has been reported by Rhone-Poulenc, ICI and Akzo. In all such cases, a mixture of sulfonated and non-sulfonated polymer is used to make hydrophilic membranes. Typical solvents used include dimethylformamide, however, there is no mention of sulfonating solvents.
In U.S. Pat. No. 4,207,182, a mixture of at least one non-sulfonated polysulfone and at least one sulfonated polysulfone is disclosed. The mixture consists of 10 to 30% by weight of a sulfonated polysulfone which has a theoretical ion exchange capacity of from 500 to 1,200 meq/kg. Semi-permeable membranes may be obtained from such mixtures using relatively concentrated solutions of the mixtures. The resulting membranes are disclosed as being particularly suitable for use in ultrafiltration.
Sulfonated polysulfone composite semipermeable membranes and a process for their production is disclosed in EP 165,077 A2. A polysulfone ultrafiltration membrane is used as the basic material to make a thin film, composite reverse osmosis membrane from a solution of previously sulfonated polysulfone by dissolving the sulfonated polysulfone and other components in a volatile solvent (e.g., ethylene glycol monomethyl ether), then letting the wet film dry to form a selective layer on the ultrafiltration membrane.
U.S. Pat. No. 4,714,725 relates to the production of asymmetric semipermeable membranes prepared using solutions of sulfonated polyaryletherketones in a solvent mixture of at least three liquids or low melting solids which are non-solvents or poor solvents for the polymer. The polymer may contain unsulfonated and sulfonated phenylene residues. The components of the solvent mixture have specified solubility parameters. Specifically disclosed is a solvent mixture formed from water, 1,4-dioxane and acetonitrile.
Synthetic hydrophilic membranes and method for their manufacture are also disclosed in U.S. Pat. No. 5,246,582. Membranes in the form of hollow fibers or flat membranes for dialysis and/or ultrafiltration are disclosed which contain a mixture of 65 to 95 wt. % sulfonated polysulfone, e.g., as a salt of sulfonic acid, and 35 to 5 wt. % unsulfonated polysulfone. Up to 12 wt. % of water-soluble polymers, based on the total mixture, are added to the mixture of polysulfone and sulfonated polysulfone before formation of the membrane.
U.S. Pat. No. 5,879,554 discloses a synthetic membrane consisting of a mixture of polysulfone, sulfonated polysulfone, and not more than 20 wt. % of other polymers, wherein the mixture contains 0.5 to 8 wt. % sulfonated polysulfone, e.g., as the salt of sulfonic acid. The method for manufacturing this synthetic membrane involves the addition of one or more solvents to the polymer mixture to form a polymer solution, which is shaped and precipitated to form a membrane in a precipitating bath by means of one or more precipitating agents.
Various disclosures address the use of acid-type solvents for making membranes. For example, U.S. Pat. No. 5,997,741 contains disclosures concerning the use of a sulfuric acid solvent for making non-sulfonated polyetheretherketone membranes.
A microporous polyetheretherketone (PEEK) membrane prepared without sulfonating the PEEK polymer is also disclosed in U.S. Pat. No. 4,992,485. Membranes, fibers and articles are prepared from solutions of PEEK in non-sulfonating acid solvents, e.g., methane sulfonic acid and trifluoromethane sulfonic acid. Use of sulfuric acid as a diluent in non-sulfonating amounts is also disclosed. The PEEK membranes are used as supports for composite ultrafiltration and reverse osmosis membranes.
Regarding the use of sulfonating solvents, e.g., concentrated sulfuric acid, for making membranes, the literature contains disclosures of the use of such solvents in the preparation of Kevlar™ membranes. In MAKU 17(2), 78-84 (in Japanese, see Chemical Abstracts Reference No. 117:28235), a method of preparing such membranes is disclosed. In the method, 1.0 to 3.0% Kevlar™ 29 is dissolved in 97% H2SO4 at 50° C. For example, 2.0% poly-phenylene terephthalamide is dissolved in 99.7% H2SO4, after which a membrane is cast from solution then moisturized and/or heated to convert the anisotropic structure to an isotropic structure, followed by a coagulation step and the removal of solvents and moisture.
U.S. Pat. No. 3,869,429 also includes disclosures relating to the preparation of Kevlar™ films using, e.g., sulfuric acid, as a dope solvent.
There is, however, no disclosure in the prior art of using acid dopes for making membranes from sulfonated polymers, more specifically, for making membrane from solely sulfonated polyethersulfones. Indeed, all references to membrane specifically concern mixtures of sulfonated polyethersulfones with polyethersulfones or the like. Moreover, there are no disclosures of only sulfonated polyethersulfones with non-aryl-sulfonate polymers.